Bitumen which is extracted from oil sands by the commercial "hot water" process as practised in the Athabasca region of Alberta, Canada contains about 4.5 wt.% sulphur and a variable proportion, up to about 20%, asphaltenes.
The bitumen is subjected to upgrading operations to form a synthetic crude oil. The initial step of such upgrading operation is to subject the bitumen to a coking step, which involves evaporating off volatiles from the bitumen to leave a solid carbonaceous material, known as "coke". The quantity of coke may vary, and is usually in the range of about 10 to 20 wt.% of the bitumen.
The coke so formed contains the asphaltenes fraction of the bitumen and also has a high sulphur content, usually about 5.5 to 6 wt.%, almost entirely of organic nature. The sulphur content of the coke inhibits its direct use as a source of thermal energy and it has previously been suggested to decrease the sulphur content of coke by reaction with hydrogen to convert the sulphur to hydrogen sulfide, and in this way render the coke more suitable for use as a fuel.
In a direct hydrodesulfurization process, the sulphur-containing coke is heated in a hydrogen stream to remove the sulphur but the extent of desulfurization obtained in this manner is usually quite poor. Such conventional hydrodesulfurization process is described, for example, in U.S. Pat. No. 3,472,622.
In my prior Canadian Pat. No. 1,098,464, there is described a process for improving the degree of desulfurization attained wherein the coke is impregnated with sodium hydroxide solution and dried prior to the hydrodesulfurization. The hydrodesulfurization is effected in a flow of hydrogen at about 700.degree. C. for about 2 hours. The sodium hydroxide, which is typically added in an amount of about 2 to 3 wt.%, appears to act catalytically and may be recovered by leaching following completion of the hydrogenation. The procedure results in over 80% of the initial sulphur being removed primarily as hydrogen sulfide.
The process of the prior invention is limited to coke which is formed by the so-called "fluid coking" process which is a continuous coking operation wherein the bitumen is sprayed onto a hot fluidized bed of coke particles maintained at a temperature of about 900.degree. F. (about 475.degree. C.). However, a so-called "delayed coking" process also is known for coking bitumen. The latter procedure is a batch one which involves heating the bitumen in coking drums at a temperature of about 800.degree. F. (425.degree. C.). The prior process is ineffective in removing sulphur from the resulting coke.
It has also previously been suggested to use sodium hydroxide oil treatment for other purposes. For example, in U.S. Pat. No. 3,179,584, it has been suggested to add sodium hydroxide or other alkali compound to oil prior to coking the same in order to increase hydrogen production. Quantities of alkali compound used range between about 1 to 20 wt.%, and where sodium hydroxide is specifically employed, a concentration of 5 wt.% is specified. This prior art is not in any way concerned with hydrodesulfurization and sulfur-containing oils are not specifically disclosed.